Magnetic random access memory (MRAM) is non-volatile memory that uses magnetism rather than electrical power to store data. FIG. 1 shows a schematic diagram of a portion 10 of an MRAM array, which includes a plurality of memory cells 12-19 and a series of conductive lines 40-48. Each memory cell 12-19 includes a magnetoresistive (MR) memory element 20-27 and a transistor 30-37. For this reason, the architecture shown in FIG. 1 is referred to as 1T1MTJ (one transistor, one MTJ) architecture.
As shown in FIG. 1, the transistors 30-33 are coupled to each other via a word line (WL1) 40, and transistors 34-37 are coupled to each other via a word line (WL2) 41, where the word lines 40, 41 form the gate electrode for the transistors 30-37. The transistors 30-33 are also coupled to each other via a program line (PL1) 42, and transistors 34-37 are coupled via a program line (PL2) 43, where the program lines 42, 43 serve as virtual ground lines. Similarly, the MR memory elements 20 and 24 are coupled to each other by bit line (BL1) 45, MR memory elements 21 and 25 are coupled to each other by bit line (BL2) 46, MR memory elements 22 and 26 are coupled to each other by bit line (BL3) 47, and MR memory elements 23 and 27 are coupled to each other by bit line (BL4) 48. The bit lines 45-48 are typically somewhat perpendicular to the word lines 40, 41 and the program lines 42, 43.
The terminology used in the art for the various conductive lines (e.g., bit lines, program lines, and word lines) can vary. For example, the “bit lines” in FIG. 1 are sometime referred to as “word lines” or “upper write lines,” and the “program lines” in FIG. 1 are sometimes referred to as “digit lines” or “bottom write lines.” For the sake of clarity, the terminology used in connection with FIG. 1 will be used throughout the present document.
Each of the MR memory elements 20-27 can be a multi-layer magnetoresistive structure, such as a magnetic tunneling junction (MTJ) or a giant magnetoresistive (GMR) structure. FIG. 2 shows an example of a typical MTJ structure 50. The MTJ structure 50 includes four basic layers: a free layer 52, a spacer 54 which serves as a tunneling barrier, a pinned layer 56, and a pinning layer 58. The free layer 52 and the pinned layer 56 are constructed of ferromagnetic material, for example cobalt-iron or nickel-cobalt-iron. The pinning layer 58 is constructed of antiferromagnetic material, for example platinum manganese. Magnetostatic coupling between the pinned layer 56 and the pinning layer 58 causes the pinned layer 56 to have a fixed magnetic moment. The free layer 52, on the other hand, has a magnetic moment that, by application of a magnetic field, can be switched between a first orientation, which is parallel to the magnetic moment of the pinned layer 56, and a second orientation, which is antiparallel to the magnetic moment of the pinned layer 56.
The spacer 54 interposes the pinned layer 56 and the free layer 52. The spacer 54 is composed of insulating material, for example aluminum oxide, magnesium oxide, or tantalum oxide. The spacer 54 is formed thin enough to allow the transfer (tunneling) of spin-aligned electrons when the magnetic moments of the free layer 52 and the pinned layer 56 are parallel. On the other hand, when the magnetic moments of the free layer 52 and the pinned layer 56 are antiparallel, the probability of electrons tunneling through the spacer 54 is reduced. This phenomenon is commonly referred to as spin-dependent tunneling (SDT).
As shown in FIG. 3, the electrical resistance through the MTJ 50 (e.g., from layer 52 to layer 58 or vice-versa) increases as the moments of the pinned and free layers become more antiparallel and decreases as they become more parallel. In an MRAM memory cell, the electrical resistance of the MTJ 50 can therefore be switched between first and second resistance values representing first and second logic states. For example, a high resistance value can represent a logic state “1” and a low resistance value can represent a logic state “0.” The logic states thus stored in a memory cell can be read by passing a sense current through the MR memory element and sensing the resistance. For example, referring back to FIG. 1, the logic state of memory cell 12 can be read by passing a sense current through bit line (BL 1) 45, activating transistor 30 via word line (WL1) 40, and sensing the current passing from (BL1) 45 through the MTJ 20 and on to program line (PL1) 42.
During a write operation, electrical current flows through a program line 42, 43 and a bit line 45-48 that intersect at the target memory cell 12-19. For example, in order to write to memory cell 13, a current is passed through program line (PL1) 42 and a current is passed through bit line (BL2) 46. The magnitude of these currents is selected such that, ideally, the resulting magnetic fields are not strong enough on their own to affect the memory state of the MR memory elements 20-23 and 25, but the combination of the two magnetic fields (at MR memory element 21) is sufficient for switching the memory state (e.g., switching the magnetic moment of the free layer 52) of the MR memory element 21.
A method of fabricating a magnetic memory device is disclosed in U.S. Pat. No. 6,713,802 to Lee. Specifically, this patent discloses the use of a damascene process to create a via connection between a top write line (corresponding to a “bit line” in FIG. 1) and the MTJ. A film (mask 128) is used as an insulating layer in which vias can be formed, and it is also used as a stop layer for a dielectric CMP process. Since this film (mask 128) is used as a stop layer for dielectric CMP, it has a minimum thickness of about 4000 angstroms in order to protect against being etched through itself. But because the top write line is used for generating a magnetic field for writing to the MTJ, it is desirable to have the write line as close to the MTJ as possible. Thus the thickness requirement of the interconnect film (mask 128) competes with the desire to place the write line closer to the MTJ.